Carmarthenshirealso called Carmarthen, Welsh Sir Gaerfyrddincounty of southwestern Wales, extending inland from the Bristol Channel. The present county is coterminous with the historic county of the same name. It rises from sea level along the Bristol Channel to an elevation of more than 2,000 feet (600 metres) at Black Mountain in the east.

On the hills above the 600-foot (180-metre) contour, there are Stone Age tumuli and cairns and, especially in the northwest, various stone monuments. Roman hilltop camps stood on spurs of the high ground. During the post-Roman centuries the Celtic saints reflected the Christian faith of the county, which suffered numerous raids by the Irish and the Scandinavians. The strong reign of Rhodri Mawr (c.870) is said to have brought a measure of peace, and his grandson Howel the Good (Hywel Dda) was the first to codify the ancient laws of Wales at his palace, Ty-Gwyn-ar-Dâf (near modern Whitland). After 1080 the Normans entered the area, built castles along the coast, and gradually penetrated the valleys to Carmarthen and Llandeilo. The campaigns of the English king Edward I gave the Normans a more unified control of the principality, and the Statutes of Rhuddlan (1284) created the county of Carmarthenshire. In 1536, under Henry VIII, the county was enlarged. The hill country, an area ideal for sheep raising, made Carmarthenshire famous for its wool in the later Middle Ages.

The town of Llandovery was strategically important in Roman times, and the nearby Church of Llanfair (restored 1915) was built within the ramparts of a Roman fort. The ruins of the 13th-century Dynevor Castle are located just west of Llandeilo. Originally built in AD 876 by Rhodri Mawr, the castle was taken by the English in the 13th century and rebuilt. The Welsh leader Owen Glendower tried to retake it in 1408 but failed. Kidwelly, established in 1115, is one of the oldest boroughs in Wales. Its castle was built by Roger of Salisbury in 1106 as one of a network of strongholds to maintain Norman control of South Wales.

The county had important connections with the house of Tudor in the 16th century through Sir Rhys ap Thomas, who fought for Henry Tudor. In the 17th century, during the English Civil Wars, the castles, especially the southern ones, offered half-hearted resistance to the Parliamentarian forces. Griffith Jones (1683–1761), vicar of Llanddowror, founded circulating schools, which effectively began the modern educational movement in Wales. William Williams Pantycelyn (1717–91), the chief hymnologist of the Welsh Methodist movement, lived and worked near Llandovery. The county was a site of the Rebecca Riots (1843), which erupted in response to increased tolls and tithes and discrimination against the poor. More recent noteworthy residents include the poet Dylan Thomas (1914–53), who lived at Laugharne and is buried there, and Gwynfor Evans, who became the first Welsh nationalist to sit in the House of Commons when he was elected to represent Carmarthen in 1966.

In the southeast, anthracite coal mining during the 19th and early 20th centuries promoted the growth of many mining villages and industrial towns, including Llanelli and Ammanford. After World War II new industries in the southeast replaced heavy industry and coal mining, which had ceased by the end of the 20th century. Today Llanelli produces tinplate for packaging and fabricated steel products, and the town has several other light manufactures.

Outside the southeast, the county remains mostly rural. Dairy farming is the main agricultural activity, and mixed farming and sheep rearing take place in the northern hills. Both Llandovery and Llandeilo are agricultural market towns, as is Carmarthen town, which is also a busy shopping centre with some milk-processing industry. Moreover, Carmarthen is the administrative centre and the historic county town (seat) of Carmarthenshire. The coastal town of Burry Port is a yachting centre. The eastern part of the county includes a section of Brecon Beacons National Park. Area 923 915 square miles (2,390 371 square km). Pop. (1998 2005 est.) 169178,000100.